standard model @ hadron colliders i basics & qcd p.mättig bergische universität wuppertal

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Standard Model @ Hadron Colliders IBasics & QCDP.MttigBergische Universitt Wuppertal

Peter Mttig, CERN Summer Students 2014Pillars of the Standard Model

Standard model:Comprehensive and precisly tested

Peter Mttig, CERN Summer Students 2014

40 years of experimental scrutinyLEP, SLC, HERA, Tevatron, PETRA, PEP, SppS, .LHC Standard model on the shoulder of giantsAnswers to questions of a century ago!Standard model pillar I: Matter

Twelve kinds of spin fermions to build the world

LHCs role:

Heaviest Standard Model particle - Top Quark:unique at hadron colliders

Peter Mttig, CERN Summer Students 2014Neutrinos not really testable @ hadron collidersMost quarks/all charged leptons very deeply scrutinized

Standard Model pillar II: Forces


realised by three Spin 1 bosonsDynamics known to a precision of 10-5 10-8

LHCs mission: probe rare processesprobe dynamics at unprecedented energies Three interactions to hold world together

Standard Model Pillar III: Higgs

Massive Bosons and FermionsBasic principles of SM violatedCross section violates unitarity

SM: new elementary boson

LHCs mission: find it !!!!!!! MISSION ACCOMPLISHED!!!!! (????)Peter Mttig, CERN Summer Students 2014Spin 0 particle to prevent theory from explodingFermion masses: for SU(2) in weak interactions6LHCs quest: New PhysicsUnderstanding SM precondition to establish New Physics Standard Model processes background Tools Standard Model to the extreme


LHC entering unchartered territory

Our dream for the next years7Todays Santa Maria: LHC

Peter Mttig, CERN Summer Students 2014Proton Proton Kollisionen@ 14 TeV c.m. energy(up to now 8 TeV) Focus on results from ATLAS and CMS

(LHCb Tim Gershon ALICE Constantinos Loizides)LHC increases energy range by factor ~ 10Precision (statistics) by factor ~100 or more Historically almost unprecedented step in energy/precision!Peter Mttig, CERN Summer Students 2014How protons interact

Proton interactions: complex events


More than 1000 particles!

Only fraction interesting

Experimenters task:- Identify those- Classify events

Reveals picture of space - time of 10-19 m 10hard scatter: two in two oute.g. gluon-gluon top-antitop


x1 * P x2 * P tt

This can be well calculatedIncoming: gluons of momenta p1, p2 Outgoing: e.g. pair of top / anti-top quarks

Characterising scattering process

Peter Mttig, CERN Summer Students 2014High Q2 probe of small distances

Quark AscatteredQuarkQuark BoutgoingQuarkgluonq

High resolution, But rare: s ~ 1/M2

Billiard tisch, Mscatter = E1*E212Add-on 1: Parton distribution function

Peter Mttig, CERN Summer Students 2014Parton energy: only fraction x of proton energyStatistical distribution: parton distribution function (pdf) FfFor X-section: all combinations (x1, x2) M contributeExample: M = 0.3 TeV from (x1=0.2, x2=0.2),(0.25, 0.16), (0.4, 0.1), ....

Ff(x) depends onkind of parton fQ2

How to derive pdfs

Peter Mttig, CERN Summer Students 2014Parton distribution functions (pdf):Not calculable from QCD, instead derive from data (ep, np, ....)

Take from theoryNot needed for lpSome dependence on Input dataCalculationParametrisationSeveral schools of pdfsCTEQ, MSTW, HERApdf .

Basic Features of pdfs


High x: mostly quarksLow x: mostly gluonsValence u, d quarks: x ~ 0.2 15Significant uncertainties


G.WattLHC processes sensitive to pdfs: Specific processes will allow to disentangle contributions some self calibration of pdfsHigh x:few gluons Large uncertaintylow x:many gluons Theoretical & experimental uncertaintiesColl and infrred safety16

Add on 2:Interaction disturbed by gluons

Peter Mttig, CERN Summer Students 2014calculable(although in most cases not completely) (N)NLO calculation,i.e. full matrix element with up to two additional partons O(as3)

Beyond this: iterative parton splitting (Markov chain) no interferences considered

Peter Mttig, CERN Summer Students 2014Quarks and Gluons turn into pions, kaons, protons:

hadronisationpartons turn to hadronsNo fundamental description model with many parameters

Peter Mttig, CERN Summer Students 2014Remainung quarks and gluons interact

Underlying eventColour flow of proton remnants

Requires multidimensional integration Monte Carlo simulationSeveral QCD generators: PYTHIA, HERWIG, SHERPA, .. (see Fabio Maltonis lectures)

Noise & Signal

20Noise & Signal at the LHC

Peter Mttig, CERN Summer Students 2014Noise: homogeneous distribution of low momentum hadrons:underlying event

Signal: two high energetic muons

Understand and subtract noise

MuonMuon21Standard Model tests


Hard scatterpdfsUnderlying eventThree contributions only mildly related (factorisation) but lead to higher uncertainties

Table22Recurring variable: Rapidity

Parton collisions not in rest system rapidity accounts for boost Peter Mttig, CERN Summer Students 2014

For m 0: y h pseudo rapidity

Typical for radiative multi-particlesDifference of rapidities Lorentz invariant

Separation is Lorentz invariantSimpler to measure, but Dh not Lorentz invariantSche23Peter Mttig, CERN Summer Students 2014Soft interactions

No direct parton parton hit


Partons see each other from distanceProton breaks into pieces fly along the beam direction

low Q2 large strong coupling as

Perturbation theory breaks down: ~ as*a + a2s* b for as > 1 divergent

Exact calculation not feasible approximate by model assumptions

25Examples of model concepts

Colour screeningPt cut -offOverlap of protons

Colour reconnectionPeter Mttig, CERN Summer Students 2014Introduces free parameters to be determined from dataSoft processes provide constraints in special conditions

Extrapolate to all topologies (and hope it will work) Challenging! Only an approximate description possible! Beispiel PYTHIA 26Separating underlying eventsUse Z0 leptons production:

Z0 has no QCD colour no part of soft interaction Soft contributions =Event - leptons

Peter Mttig, CERN Summer Students 2014Schematic approachEvent =hard parton parton scatter +soft contributions

27Particles in underlying events

Different models and different parameters different predictionsModel comparison to data best model to describe underlying event Peter Mttig, CERN Summer Students 2014

2899,999% of all LHC events: soft

no trigger bias minimum bias events

At 7 TeV: 6 charged particles per |Dh | = 1, mostly low pTModel parameters have to be adjustedNote: per LHC bunch crossing ~ currently 30 of these events (pile up events) the price of high hard event ratePeter Mttig, CERN Summer Students 2014

29Repeat notation for soft events

Peter Mttig, CERN Summer Students 2014Underlying Event:collision of two specific protons one hard parton interactionUE: part of event that does not take part in hard interaction

Minimum Bias Event:(overwhelmingly) event with no hard interaction

Pile Up Events:Within same bunch crossing: several proton interactionsOne of them leads to hard parton interactionOthers are minimum bias like pile up events30Peter Mttig, CERN Summer Students 2014Parton jets hard QCD

Hard interaction: Jets

Peter Mttig, CERN Summer Students 201432Jets are universal

Jets: bundles of hadrons representative of quarks and gluonsdirection, energy + (sometimes) parton flavour measurabledirect QCD tests possibleExperimental challenge: extract jets from 1000 particlesPeter Mttig, CERN Summer Students 2014

e+ e- collisionse p collisions33Jets are even more universalPeter Mttig, CERN Summer Students 2014

34How to find a jet ?

Unambiguous connection to underlying partons Comparison to theoryAnyway ..... how many jets?broadness of jets arbitraryjet multiplicity depends on choicedefined according to physics

Predefine how broad a jet should be!


35Sequential jet finder

Reverse evolution of event

Select one particle (e.g. most energetic)Find most similar particle, (e.g. smallest angle, pt)Is combination smaller than predefined cut off value (e.g. maximum angle, maximum mass ....) IF YES:4 Combine to a new pseudo particle (e.g. sum 4 momenta)5 Go to 2

IF NO:4 Jet found: sum of all associated particles Start next jetPeter Mttig, CERN Summer Students 2014Coll and infrred safety36Standard jet finding at LHC: Anti kt

Gavin Salam

Select hard particles as seeds for jets: favouredby min(pt-2)

Hard particles separated in space are disctinct seeds: large DRij cut off given by dij (steered by R)Peter Mttig, CERN Summer Students 2014

Coll and infrred safety37

Standard jet finding at LHC: Anti kt Gavin Salam

Gradual dij increase:Associate close by particles:mostly soft ones in neighbourhood

(if no hard ones close by)

Continue until Rmax reached Peter Mttig, CERN Summer Students 201438

The final jets Gavin SalamAll particles assigned to jets

Close to circular in spacegood for experimental corrections

Note: special treatment of particles close to beam

Typical DR 0.4 0.6Peter Mttig, CERN Summer Students 2014

39Peter Mttig, CERN Summer Students 2014Probing the strong force

The key test: jet cross section

Calculated to Next-to-leading order (NLO)Peter Mttig, CERN Summer Students 2014

Steeply falling

7 orders of magnitude between 20 500 GeV

Significant tests require high experimental precision41Experimental challenge I: JES

Jet energy determined from calorimeter (+ tracking information)How well is scale known? Jet energy scale (JES) uncertainty Effects on X-section magnified by steep slopeCalibration: g + jets data with pT balanceJet ptLog sTrue cross sectionObserved distribution dueto wrong energy scale

Jet energy scale known to 1 3%!3% at low pT: jet/photon id2% at high pT: statistics

Peter Mttig, CERN Summer Students 201442Experimental challenge II: JER

sensitivity of measured cross section to energy resolutionJet ptLog sTrue cross sectionExptl. resolutionJet ptLog sObserved dueto energy resolutionPeter Mttig, CERN Summer Students 2014Use pT balance in 2 jet events

Discuss trigger prescaling,Importance of JES and resolution43Uncertainties: summary

Experimental uncertaintiesdominate at low pT

pdf/theoretical uncertaintiesalmost as high at high pT


Discuss trigger prescaling,Importance of JES and resolution44LHC QCD studies

Peter Mttig, CERN Summer Students 2014 QCD correct at multi TeV scales?

Constrain basic QCD parameters

Jets understood as tool for high pT physics 45Jet cross sections in rapidity and pT

Excellent agreementtheory data

over huge range inphase space:10 orders of magnitude

Jet pT up to 2 TeV Peter Mttig, CERN Summer Students 2014

46 jet jet mass


Excellent agreementtheory data

Probing masses upto 5 TeV!

Forward jets yield higher masses

47Determining the strong coupling as


Single value less precise, but huge energy rangeEnergy dependence of as clearly visible

asMeasures of as :Three jet fractionJet mass

Discuss trigger prescaling,Importance of JES and resolution48Measuring jet evolution

Peter Mttig, CERN Summer Students 2014Very good agreement . at least for some models

our view of jet evolution:Sequential radiation of gluonsLeading to finer granularities

Mimicked in models how good are these models?

Measure, at which dij jet is split into two, three, ..I.e. reverse jet findingInto 2 jets .Into 3 jets .Ratio 2/3 jets split Importance of JES and resolution49High pT Jets

I.e. single jet has higher massMeasure mass of high pT jet:

Globally: models agree with expectation ..... But details differ! Peter Mttig, CERN Summer Students 2014High pT jets: important to explore TeV scale physics

Lorentz BoostSeveral partons merge into a single jet50Peter Mttig, CERN Summer Students 2014Are quarks composite?

Are partons composite?

pp interaction @ Q 0.1 GeV qq interaction @ Q 10 GeV interaction of subconstituents ? @ Q 1000 GeV ?


Rutherford all over again .

52Jets and BSM:Search for di jet resonances

An excited quark ?q* q + g(Remember excited atom, nucleus ....)

Would be strong signal for compositeness

Search for resonance:enhancement Peter Mttig, CERN Summer Students 2014

53

Are quarks composite ?

Compositeness modify angular distribution

continous change with higherjet jet mass

No deviation from SM observed:L > 7.8 TeVPeter Mttig, CERN Summer Students 2014

Note: results applicable to other exotic models: black holes, colour octet quarks, ......54Peter Mttig, CERN Summer Students 2014Strong interactions at core of pp interactions

Multihadron events (soft interactions) measured Jet cross sections agree with predictions over a wide range Probing Multi - TeV range: no sign for physics Beyond Standard ModelPeter Mttig, CERN Summer Students 2014QCD effects: number of jets

Even though not exact matrix element:Good agreement on jet multiplicityPeter Mttig, CERN Summer Students 2014The lighter (orange) shaded error bands correspond to the systematic uncertainties on the measurement, excluding the luminosity uncertainty. The darker (grey) shaded error band corresponds to the systematic uncertainty on the the measurement, normalized to the inclusive two-jet multiplicity bin. A plot of the ratio of the different Monte Carlo simulations to the data is presented at the bottom of the figure. 57

standard model @ hadron colliders i basics & qcd p.mättig bergische universität wuppertal

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